This invention relates to clay compositions and objects including clay compositions.
Clay is an earthy substance that is composed mainly of fine particles of hydrous aluminum silicates and other minerals and is plastic when moist but becomes hard when heated to maturity. A clay object is heated to maturity by heating it for a sufficient time and to a sufficient temperature that the clay object undergoes a ceramic change and, after cooling, the clay object is hard and vitreous. Commonly, a clay object is bisque fired before being fired to maturity. In bisque firing, water and organic materials present in the clay are driven off. Bisque firing results in an object that is hard and retains its structural form when handled but is not vitreous.
Maturity is not characterized by a single temperature value but can be achieved by heating to a temperature within a range of values. For example, a clay for which the nominal temperature of maturity is 1500xc2x0 F. might be brought to maturity by heating for a suitable time to any temperature in the range from 1400xc2x0 F. to 1600xc2x0 F.
In this specification, the term xe2x80x9cfiringxe2x80x9d and derivatives is used to refer to heating to maturity unless the context clearly indicates otherwise.
The term xe2x80x9cceramicxe2x80x9d is widely used to refer to clay that has been fired, but in this specification the term xe2x80x9cclayxe2x80x9d may be used to refer both to the fired material and the unfired material. xe2x80x9cWarexe2x80x9d is a generic term that covers both green (unfired) clay and ceramic.
Fired clay is widely used in construction of artistic objects. The artistic qualities of fired clay include solidity and sculptability. Also, fired clay provides a surface for receiving glazes. A fired clay object is generally made by shaping or molding green clay and then firing the clay object in a kiln and allowing the object to cool.
The composition of a clay is frequently defined on the oxide basis, which specifies the relative proportions by weight of the oxides of the elementary ingredients. A clay body is a blend of two or more naturally occurring clays. A clay supplier may blend two or more clays from respective mines to create a proprietary clay body containing desired proportions of different respective mineral ingredients. Commercially available ceramic software can be used to design a clay body to achieve a desired oxide composition. Such software enables a ceramist or a clay supplier to select specific mined clays and the proportions of each that must be blended to provide a desired oxide composition.
Glass is widely used in construction of artistic objects. An artist may purchase glass from manufacturers in a standard form, such as chunks, sheets, stringers and frit, and construct an artistic object by heating pieces of glass in a kiln so that the pieces melt and fuse together. Typically, glass fuses at a temperature of about 1325-1450xc2x0 F. The artist may also work or manipulate the molten glass. When the molten glass has been brought to the desired form, the artist allows the glass to cool so that the form is fixed. Particularly in construction of artistic objects, it may be desirable to join different pieces of glass together, e.g. pieces of different color, to achieve a particular artistic effect.
If two glass pieces are fused together to form a composite structure, i.e. a structure having two or more distinct components, and the composite structure is allowed to cool to room temperature, there is a possibility that stresses created during the cooling process will lead to fracture of the composite structure. Accordingly, cooling must also take place in a controlled fashion to prevent or limit establishment of stresses in the glass due to improper annealing.
Two glasses are said to be compatible if they can be fused together and after proper cooling of the composite structure to room temperature there are no residual stresses that will lead to fracturing of the composite structure. Compatibility of two glasses is a function of both the viscosity (resistance to flow) of the glasses and the thermal expansion of the glasses (change in size as temperature changes). Thermal expansion affects compatibility below the strain point and viscosity affects compatibility from the strain point to the annealing point. Two glasses can be compatible if the stress caused by mismatch in viscosity is cancelled out by the stress due to mismatch in thermal expansion. For example, if the viscosity difference between the two glasses results in a tensile force and the thermal expansion difference results in compressive force of equal magnitude, the two forces cancel each other out and the glasses are compatible.
Thermal expansion of a material is characterized by the coefficient of thermal expansion (COE) of the material. The COE is the proportional change in linear dimension per degree Celsius change in temperature. For art glasses (that is, glasses that are manufactured and sold for fusing), the COE is typically in the range from about 80xc3x9710xe2x88x927 to 100xc3x9710xe2x88x927 per degree Celsius. It is common to ignore the 10xe2x88x927 factor in the COE and refer to a value of 80xc3x9710xe2x88x927 per degree C simply as a COE of 80.
The artistic qualities of glasses are different from the artistic qualities of fired clay. Glasses have the qualities of transparency and clarity of color and may have fluid lines, as compared to the solidity and sculptability of clay. These differences in artistic qualities are such that many artists wish to construct artistic objects in which clay and glass are fused together. However, hitherto it has not been possible to achieve this goal due to incompatibility of clay with glass.
Glazes that are applied to a clay object include glasses, but glazes are applied only in thin layers to the surface of the clay object and it is relatively easy to ensure that the stresses that are generated in the thin layers on cooling will not result in damage to the glaze or the clay object.
A number of glass manufacturers make a range of compatible glasses, but at present glasses that are compatible with clays used in making artistic ceramic objects are not commercially available.
A number of clay composite materials have been developed for use in scientific instruments and the electronics industry. These materials have been developed so that they have low COE values and therefore they are not compatible with commercially available glasses used for making artistic works.
In accordance with the present invention there is provided a clay composition comprising, in parts by weight on oxide basis:
and 10-20% metal oxides of the form RO or R2O.
In accordance with the present invention there is further provided a composite structure comprising a clay component and a glass component fused together, the clay component comprising, in parts by weight on oxide basis:
and 10-20% metal oxides of the form RO or R2O.
In accordance with the present invention there is also provided a ceramic object made by firing a clay composition comprising, in parts by weight on oxide basis:
and 10-20% metal oxides of the form RO or R2O.
In accordance with the present invention there is moreover provided a method of manufacturing a glass-ceramic composite structure, comprising providing a clay object and a glass object, the clay object comprising, in parts by weight on oxide basis:
and 10-20% metal oxides of the form RO or R2O, and firing the clay object in contact with the glass object.
In accordance with the present invention there is in addition provided a method of manufacturing a composite structure comprising a clay component and a glass component, the method comprising providing a clay object and a glass object, the clay object comprising in parts by weight on oxide basis:
and 10-20% metal oxides of the form RO or R2O, firing the clay object to create a ceramic object, allowing the ceramic object to cool to a temperature below a fusing temperature of the glass object, placing the ceramic object and the glass object in contact, and heating the glass object and the ceramic object to the fusing temperature.